A cyclotron is a type of particle accelerators which comprise a vacuum enclosure in which charged particles are accelerated outwards from a central axis and along a spiral trajectory in an acceleration region of a median plane of the cyclotron under the combined effect of a high frequency electric field ({right arrow over (E)}) and of a main magnetic field ({right arrow over (B)}), the latter being generated by excitation of a main coil assembly.
It is known that the main magnetic field ({right arrow over (B)}) has to be oriented as perpendicular as possible to the median plane in said particle acceleration region, in order to keep the charged particles within their desired trajectory. It is further also known that the main magnetic field ({right arrow over (B)}) has to be centred as well as can be with respect to the central axis of the cyclotron, said central axis being perpendicular to the median plane.
There is thus a need to position the main coil assembly as accurately as possible with respect to said median plane and with respect to said central axis in order to obtain the desired orientation and symmetry of the main magnetic field ({right arrow over (B)}) in the particle acceleration region.
This need is of particular importance in case the direction and amplitude of the main magnetic field ({right arrow over (B)}) in the particle acceleration region is dominated by the orientation and position of the main coil assembly, such as for example when main coil assembly comprises superconducting coils which are used to produce a magnetic field exceeding the saturation state of a ferromagnetic core which they surround or when no ferromagnetic core is used.
A method for adjusting the position of a superconducting main coil in a cyclotron is known from Dey et al. (“Coil centering of the Kolkata superconducting cyclotron magnet”; Cyclotrons and Their Applications 2007, Eighteenth International Conference). They propose to measure the forces in a plurality of support links supporting the excited main coil assembly in a hanging fashion into the cyclotron, and to centre the main coil assembly by adjusting the length of these support links in function of a lowest force criterion. After getting a minimum force position of the main coil assembly, further adjustment of the position of the main coil assembly is performed by measuring the main magnetic field ({right arrow over (B)}) in the particle acceleration region and by minimizing the first harmonic component of this main magnetic field.
A problem with such a method is that any asymmetry in the magnetic circuit will negatively influence the accuracy of the method. Another problem is that it requires sensors and related equipment for measuring the forces in all the support links, which adds complexity and cost. Yet another problem is that it is an indirect method, which may also negatively influence its accuracy.
Another known method consists in measuring the efficiency of the cyclotron when in operation and to adjust the position of the main coil assembly in order to maximize the efficiency. Indeed, when the main coil assembly is misaligned, charged particles will move out of their desired trajectory and will be lost, so that the efficiency of the cyclotron will drop and vice-versa. A problem with this method is that the efficiency may be influenced by other parameters than the position of the main coil assembly, so that this method is not accurate enough.
Although these know methods do work, there is room for improvement, particularly as far as the accuracy of the positioning of the main coil assembly with respect to the median plane is concerned.